A developed exhaust control apparatus provided in an exhaust passage of an internal combustion engine has a selective catalytic reduction NOx catalyst (hereinafter may be referred to as “SCR catalyst”) of selectively reducing NOx in the exhaust gas that is supported in a filter. The filter traps particulate matter (hereinafter referred to as “PM”) in the exhaust gas. The SCR catalyst reduces NOx in the exhaust gas using ammonia (NH3) as a reducing agent. In the description below, the filter having this SCR catalyst supported therein is called “SCRF”. In an exhaust control system for an internal combustion engine having an SCRF, an oxidation catalyst and a urea supply valve for NOx reduction are placed on the upstream side of the SCRF. Employing the SCRF in the exhaust control system allows the SCR catalyst to be placed on the more upstream side in the exhaust passage. The SCR catalyst is thus more likely to be heated by the heat of the exhaust gas. This enhances the warm-up of the SCR catalyst and improves the NOx reduction rate of the SCR catalyst.
An exhaust control system described in Patent Literature 1 has an SCRF and uses a PM sensor placed on the downstream side of the filter to determine a damage of the filter. In the case where the filter is damaged, the flow of the exhaust gas is concentrated into the damage location. This significantly increases the detection value of this PM sensor. A damage of the SCRF is determined by using such an increase in the detection value.